Descaling metals



i it 1 DESCALING NIETALS Giles Carter, Niagara Falls, N. Y., assignor to E. I. du Pont de Nemours and Company, Wilmington, Del., a corporation of Delaware No Drawing. Application October 13, 1954, Serial No. 462,110

18 Claims. (Cl. 252156) methods for maintaining the bath in an active state and for descaling metals therein.

The above-mentioned and still further objects are achieved in accordance with this invention by a process in which metal objects to be descalcd are first immersed for a few minutes in a molten bath of an alkali metal hydroxide containing a carbide of the alkali metal, then withdrawn and finally quenched in water. Additional treatment may then beapplied to the metal as, for example, a wash in a high-pressure water spray or a rinse in dilute acid. Base metal so treated is then ready for electroplating, painting or other operation as desired.

The preferred alkali metal hydroxide and carbide for use in this invention are those of sodium since they are relatively cheap, but the corresponding compounds of some other elements of group I of the periodic system may be used as well. Potassium compounds, for. example, may be substituted in whole or in part for those of sodium.

The carbide employed should in general constitute up to 20% by weight of the molten bath. Since as good results inv descaling are obtained with 1% of the carbide as with 20%, the upper limit is not sharply critical. Baths containing the higher carbide concentrations may require higher fusion temperatures. A convenient lower concentration limit may be set at around 0.5% by weight but even smaller percentages of carbide provide some descaling. Terminology such as an effective amount of carbide may be utilized to designate any quantity suitable for descaling.

The temperature employed for the descaling may vary within rather wide limits from a minimum set automatically by the melting point ofv the bath. A temperature of between about 325 and 625 C. will generally be found satisfactory while 350425 C. is preferred. Higher temperatures may be employed, if it is so desired, but they confer no special benefits and are uneconomical.

The time required for cleaning the metal will depend upon the thickness of the scale present. Around 2or 3 minutes will suffice for the reduction of thin scale, while around minutes are sufficient for about any thickness normally encountered. As the bath does not affect ferrous metals, over-exposure of the latter to the carbidecaustic mixture is immaterial.

The present descaling baths may be formed and sodium carbide maintained therein in any convenient manner. Preformed sodium carbide may, for example, be added States PatentO" 2. to the caustic from some external source. Since there are few known methods of preparing sodium carbide, that of co-pending application S. N. 388,072, filed October 23, 1953, and of common assignment herewith may be mentioned. This method involves reacting molten caustic with carbon at around 700 C. or above. The entire product may be added to the bath since. the other compound formed by the reaction, sodium carbonate, does not interfere with the descaling operation.

The carbide may also be prepared directly in the bath. One method of accomplishing this result is by taking advantage of the reaction between caustic and carbon noted above and merely adding powdered charcoal or graphite to caustic heated to 700. C. or above. The small amount of by-product hydrogen formed may actually aid in reducing oxide scale. Only a small quantity of carbon should be introduced into the bath at one time, sufficient to form a carbide constituting only about 1% by weight of the bath being preferred. The reaction to form the carbide goes to completion very slowly; some of the carbon usually remains unreacted on the surface and serves as an insulating layer protecting the bath from the atmosphere. Generation of the carbide in the bath by reaction between carbon and caustic is less preferred than some other methods because the temperature involved, i. e. at least 700 C., may be too high for the bath containers normally employed.

. Another method of forming the sodium carbide in situ is by reacting sodium oxide with acetylene. When this procedure is adopted, sodium oxide is added as such or sodium in the brick or other solid form is introduced onto ployed away from the bath and the carbide produced added to the bath as such, if it is so desired.

Sodium carbide may also be produced in the bath from calcium carbide and molten caustic. This method, however, possesses the disadvantages that it entails the formation of calcium oxide and appreciable loss of acetylene. Calcium oxide precipitates at the temperatures generally employed and results in an undesirable sludge in the bath. The reaction producing the carbide is believed to be:

It will be understood that enumeration of a few methods of maintaining the sodium carbide concentration in the present baths does not preclude the use of others. The carbide may, in fact, be produced by means of any reaction yielding the desired product in usable form. If the reaction is suitable, it may be accomplished in the bath itself. In any case the reaction may be carried out apart from the bath and the carbide added thereto in a separate operation.

In one modification of this invention a small amount of sodium oxide is added to or maintained in the molten caustic along with the sodium carbide. Scale reduced in a bath prepared in this maner is completely and easily removed in the water quench and the resultant metal is brighter in appearance than metal treated in a bath con taining no oxide. Between about 1 and 6% by weight of the oxide is sufiicient to accomplish this result. Sodium oxide is produced upon the addition of metallic sodium to molten caustic and may be present whenever the carbide is formedby reaction between sodium and acetylene within-the caustic.

It will be understood that the bath may contain materials other than alkali metal hydroxide, carbide and oxide. The carbonate has already been mentioned as compatible Patented May 28, 1957 to alter the melting point if it is so desired. The major requirement is that the additional salt not react with other components of the bath. Salts of different alkali metals may be used in the same bath for special purposes.

The exact mechanism by which the carbide removes the oxide scale is unknown. It is believed, however, that the reaction may essentially be represented by the equation:

The iron formed by chemical reduction of the scale is usually found on the base metal as finely divided particles easily removed, at least in part, by the subsequent water quench. The finely divided material is very porous and easily penetrated. Consequently oxide scale of almost any thickness can be, reduced as described.

The presence of carbide in the bath may be established by treating a cooled sample of the bath with water and bubbling the evolved gas through an ammoniacal solution of cuprous chloride. A quantitative determination may be made by weighing the copper in the precipitate produced as described from a known sample of bath and calculating the carbide by means of the obvious stoichiometrical equivalents. The active content of the bath, i. e., Nazca and any NaH accidentally formed, may be determined by measuring the gas evolved from the interaction of a weighed sample of the bath and an amount of water sufficient to dissolve the sample.

The bath of this invention is particularly useful in descaling ferrous bases such as carbon and stainless steels. It may likewise be used to clean various non-ferrous metals and their alloys including, for example, copper and nickel and alloys thereof and Nichrome and other alloys used as electrical resistors. In fact, almost any metal that can withstand the high alkalinity of the molten caustic can be treated in carbide baths. The bath may also be used to produce fine iron powders from iron oxides.

An important advantage of my bath is that it does not deteriorate but may be employed indefinitely. The reduced iron is largely removed from the bath on the workpiece from which it was produced. Build-up of foreign matter is consequently long delayed. It is, of course, necessary to replace carbide and caustic as they are exhausted or carried out of the bath with the descaled metal.

Another advantage of the bath is that the carbide utilized may generate a thin layer of carbon, on. the surface of the bath. This carbon layer seals the molten caustic from the atmosphere and reduces interaction between the carbide and active constituents of air such as oxygen, water vapor and carbon dioxide. In the pre ferred embodiment of the invention a thin layer of charcoal is deliberately placed on the bath to minimize contact with air. An inert atmosphere as, for example, one of nitrogen will serve the same purpose as the carbon layer but may be inconvenient.

A further advantage of this invention is that the carbide bath need not be maintained by generation of the active ingredients in situ from gases although, optionally, it can be so maintained. Some other descaling baths, notably that based on sodium hydride, require generation of the descaling agent in the bath since the agent employed is too unstable to be handled alone. Control of the hydrogen or ammonia required for the formation of sodium hydride presents diificulties avoided entirely when relatively stable sodium carbide is introduced from an external source or generated in place from solids such as sodium oxide and carbon.

A still further advantage of this invention is that it can provide a reducing bath which contains no free hydrogen. Such a bath is of importance in descaling metals subject to hydrogen embrittlement. Whenever free hydrogen is unwanted, itwill be understood that methods '4 of forming or replenishing the carbide in the bath which simultaneously generate hydrogen must be avoided.

There follow some examples which illustrate the present invention in more detail. In these examples, unless otherwise noted, all percentages are by weight and all pressures are substantially atmospheric.

Example 1 This example shows use of a carbide-caustic molten bath in descaling iron.

Sodium carbide powder was prepared by reacting sodium and acetylene in a modification of the process of Guernsey and Sherman, J. Am. Chem. Soc. 48, 141-146 (1926). The carbide was then mixed with a small amount of molten sodium hydroxide in a rotary reactor at 350 C. The resultant mixture was allowed to freeze, forming a dense solid which adhered to the walls of the reactor. It was easily cracked off the walls and stored conveniently in a closed glass bottle until needed. It was then gradually added as desired to a batch of molten sodium hydroxide, under a nitrogen atmosphere, utilized as a descaling bath.

Tests on steel rods covered with scale were commenced when the carbide concentration reached 0.5% and were continued until the concentration was 7.5%. In each test the iron piece was dipped into the bath for from 2. to 60 minutes and then quenched in water. Reduced scale formed in this bath was largely but never entirely removed by the quench.

Variations in carbide concentration over the range and in the time indicated had no great effect on the removal of the scale, at least partially complete in each test.

The temperature of the bath was varied from 350 C. to 550 C., likewise with no effect on scale removal.

Since sodium carbonate is usually formed in these baths, it was gradually added until a concentration of 30% was reached. No effect on the descaling was produced by the change in carbonate concentration.

Example 2 This example shows the use of sodium oxide in the carbide bath.

A bath was made up byadding 1-2% of preformed sodium carbide and 24% sodium oxide to molten sodium hydroxide under a nitrogen atmosphere. The bath was analyzed periodically for carbide and for iron concentration. The iron in solution appeared to be proportional to the oxide concentration. The bath was maintained by additions of sodium carbide and a mixture of sodium oxide and ferric oxide. The bath was noncorrosive under these conditions and remained effective for the entire test period.

The bath was opened to the air and numerous steel rods covered with scale were dipped thereinto at intervals during a test period of over three weeks. The temperature of the bath was maintained at 400 C. After a dip of 10 minutes each rod was withdrawn and given a water quench. The scale was completely reduced by the immersion in carbide and the reduced scale was generally entirely removed by the water quench. Rods subjected to this procedure were consequently cleaner and brighter than those descaled, by the method of Example 1.

Example 3' Samples of scaled iron were descalecl in the bath atv intervals over a period of a week. Bright metal was generally produced as in Example 2 above.

Having described my invention, I claim:

1. The process for removing metal oxide from the surface of a metal article formed from a metal substantially non-reactive with alkali metal hydroxides which comprises contacting saidarticle at a temperature below the melting point thereof with a molten composition comprising an alkali metal hydroxide of the group consisting of sodium and potassium hydroxides having up to about by weight of a carbide of a member of the group consisting of sodium and potassium dissolved therein and subsequently quenching said article with water.

2. The process of claim 1 in which the molten composition comprises additionally up to about 6% by weight of alkali metal oxide.

3. The process for removing metal oxide from the surface of a metal article formed from a metal substantially non-reactive with sodium hydroxide which comprises contacting said article at a temperature below the melting point thereof with molten sodium hydroxide containing about 05-20% by weight of sodium carbide.

4. The process of claim 3 in which the molten sodium hydroxide contains additionally up to 6% by Weight of sodium oxide.

5. The process for cleaning a ferrous metal article to remove metal oxide therefrom which comprises immersing said article in a molten composition comprising sodium hydroxide containing 05-20% by Weight of sodium carbide and subsequently quenching said article in water.

6. The process of claim 5 in which the molten sodium hydroxide contains additionally up to 6% by weight of sodium oxide.

7. The process of claim 5 in which the temperature of the molten composition is 325650 C.

8. The process for reducing an oxide of a metal substantially non-reactive with molten alkali metal hydroxides to convert said oxide to the corresponding metal which comprises contacting said oxide with a molten composition comprising an alkali metal hydroxide of the group consisting of sodium and potassium hydroxides containing 05-20% by weight of a dissolved carbide of an alkali metal of the group consisting of sodium and potassium.

9. The process of claim 8 in which the molten composition comprises additionally up to 6% by weight of the oxide of said akali metal.

10. The process of reducing an iron oxide to metallic iron which comprises contacting said oxide with molten sodium hydroxide containing 05-20% by weight of sodium carbide.

11. In a process for cleaning a plurality of ferrous articles carrying a surface oxide scale by immersion thereof in fused sodium hydroxide containing sodium carbide, the step comprising maintaining the concentration of said sodium carbide at 05-20% by Weight of the hydroxide.

12. The process of claim 11 in which the concentration of the sodium carbide is maintained by periodic addition of preformed sodium carbide to the fused hydroxide.

13. The process of claim 11 in which the concentration of the sodium carbide is maintained by generation of the sodium carbide in situ Within the fused sodium hydroxide.

14. The process of claim 13 in which the sodium carbide is generated in situ by reaction between sodium oxide and a hydrocarbon.

15. In a process for cleaning a plurality of ferrous articles carrying a surface oxide scale by immersion thereof in fused sodium hydroxide containing some sodium carbide and sodium oxide, the step comprising passing acetylene gas through said fused sodium hydroxide and thereby generating sodium carbide in situ by reaction between sodium oxide and acetylene.

16. A fused bath for removing metal oxide from metal articles formed from metals substantially non-reactive with molten alkali metal hydroxide which comprises the molten hydroxide of an alkali metal of the group consisting of sodium and potassium containing about 05-20% by weight of the carbide of said metal dissolved therein.

17. A fused bath for removing metal oxide from metal articles formed from metals substantially non-reactive with molten sodium hydroxide which comprises molten sodium hydroxide having about 05-20% by weight of sodium carbide dissolved therein.

18. A fused bath for removing metal oxide from metal articles formed from metals substantially non-reactive with molten sodium hydroxide which comprises molten sodium hydroxide having about 05-20% by weight of sodium carbide and up to 6% by weight of sodium oxide dissolved therein.

References Cited in the file of this patent UNITED STATES PATENTS 2,211,047 Beck Aug. 13, 1940 2,254,328 Steigerwald Sept. 2, 1941 2,377,876 Gilbert June 12, 1945 2,431,479 Holden Nov. 25, 1947 2,601,864 Holden July 1, 1952 

3. THE PROCESS FOR REMOVING METAL OXIDE FROM THE SURFACE OF METAL ARTICLE FORMED FROM A METAL SUBSTANTIALLY NON-REACTIVE WITH SODIUM HYDROXIDE WHICH COMPRISES CONTACTING SAID ARTICLE AT A EMTPERATURE BELOW THE MELTING POINT THEROF WITH MOLTEN SODIUM HYDROXIDE CONTAINING ABOUT 0.5-20% BY WEIGHT OF SODIUM CARBIDE.
 16. A FUSED BATH FOR REMOVING METAL OXIDE FROM METAL ARTICLES FORMED FROM METALS SUBSTANTIALLY NON-REACTIVE WITH MOLTEN ALKALI METAL HYDROXIDE WHICH COMPRISES THE MOLTEN HYDROXIDE OF AN ALKALI METAL OF THE GROUP CONSISTING OF SODIUM AND POTASSIUM CONTAINING ABOUT 0.5-20% BY WEIGHT OF SAID METAL DISSOLVED THEREIN. 